Within baseball, the term bat speed is defined as how fast a bat moves through its arc when a batter swings it. Bat speed is generally recorded as the speed from the bat's center of mass, which is near its "sweet spot" (the most effective section with which to hit a ball). A batter has about one-half second to swing the bat from the time the ball is released by the pitcher to the time it enters the catcher's mitt—what is called swing time.
When considering the mechanics of the bat swing, researchers often model it based on its similarity with how a person swings a weight at the end of a rope. In both cases, the forces on the swung object are exerted through the person's hands and directed along the axis of the object. Thus, the bat speed can be determined as the swing proceeds through a series of motions involving first the hips, and then the shoulders, arms, and lastly the wrists, as the bat is driven powerfully around from the rear to the front of the batter.
Before the pitch, the batter usually stands congruently above a point halfway between his or her feet. As the ball is released by the pitcher, the batter shifts his or her weight backward by rotating (or cocking) the hips toward the rear foot. The batter then steps into the pitched ball by pushing forward off his or her rear foot. A typical major league player pushes off with a force of around 250 pounds-force (1,110 newtons). During this initial period of about 0.2 seconds, from which the shoulders are brought around roughly parallel with the hips, the bat has reached a speed of about 6 mph (10 km/h).
The batter then begins to rotate his or her body around the fixed front foot. As the batter continues to rotate his or her body approximately within a horizontal plane, the bat is brought across the plate with the arms. During this stage of the swing, which lasts about 0.08 seconds, the hands transmit increasing amounts of force to the bat as the body continues to rotate while pulling the bat through its arc—eventually reaching nearly 50 pounds-force (220 newtons). During the last 0.01 seconds of this period, the bat speed reaches about 20 mph (32 km/h).
In the 0.06 seconds left for the batter to complete the bat swing across the plate, most of the energy has been transferred to the bat from the thigh and torso muscles (with the arm and hand muscles serving primarily as a way to transfer energy). At this point, the force on the hands and arms attain a value of about 200 pounds-force (890 newtons). Just before the bat meets the ball, the wrists are straightened out as the remaining energy is transferred to the bat. About 0.04 seconds before the bat hits the ball, the bat speed reaches about 70 mph (113 km/h).
If the ball is struck properly on the sweet spot, about 50% of the energy stored within the swinging bat is transferred to the ball. The speed of the bat is quickly reduced by approximately 30% from the reaction of the collision—to a speed of about 50 mph (80 km/h).